It is known to perform ECG-controlled CT multirow spiral recordings of the heart. In this case, the patient's ECG signal is concomitantly recorded during the recording of the multirow spiral data record. The ECG signal is used in order later, during the image reconstruction of CT images, to select measurement data. Thereby, for each image, a contribution is made only by measurement data which have been recorded in specific phases of the cardiac cycle that can be chosen by the user.
Thus, by way of example, there may be the requirement to reconstruct images in the diastolic resting phase. This can be done in order to obtain an imaging of the coronary arteries that is not disturbed by motion artifacts. The temporal positions of the R spikes can be used as a simple reference point in the ECG signal. For each CT image, the intention then is to use for image reconstruction e.g. only data which have been recorded in a certain time window with a specific relative spacing from the preceding R spike (e.g. measured in % of the duration of the RR interval of the ECG signal).
Such a recording technique with associated multirow spiral image reconstruction is the subject matter of DE 198 42 238 A1.
In order that the examination object to be imaged can be expediently reconstructed, measurement data records at successive projection angles α are necessary which extend in parallel geometry over a reconstruction interval of at least 180° (reconstruction interval [αmin, αmax]≧180 °). Depending on the desired temporal resolution in an image, the total reconstruction interval [αmin, αmax] is composed of n data intervals which have been recorded in successive cardiac cycles with respect in each case to the same relative cardiac phase, as is illustrated in FIG. 1.
Given a rotation time Trot of the CT scanner of 500 msec, for example, achieving a temporal resolution of 50 msec in the individual image in the best case requires at least n=5 data intervals each of 36° comprising n=5 successive cardiac cycles for image reconstruction. The table advance speed v in the z direction (the z direction is the direction of the longitudinal axis of the patient) during the spiral recording must in this case be chosen to be so small that the multirow detector continues to move by at most a total detector width D during the n successive cardiac cycles. This is because it is only then that each z position of the examination object is irradiated during the n cardiac cycles required for image reconstruction, and all the measurement data required for image reconstruction can be obtained at each z position in the manner revealed in FIG. 2.
At low heart rates, this leads to such small table advance speeds that, during the customary maximum time for a spiral recording given by the patient's breath holding time, only inadequate object lengths can be covered at the required resolution in the z direction (layer thickness). Given a heart rate of 70 beats per minute, n=5 successive cardiac cycles last about 4.3 sec.
Assuming a multirow detector with 4 detector rows which each cover 1 mm in the z direction (layer thickness 1 mm, total detector width D=4 mm), then the detector is permitted to continue to move precisely 4 mm during 4.3 sec. In a customary breath holding phase of 35 sec, it is thus possible to cover at most 32 mm, which is far too little for imaging the heart.
In order to increase the object length that can be covered during a breath holding phase, it is either possible to choose a larger layer thickness (e.g. layer thickness 2.5 mm, total detector width D=4*2.5 mm=10 mm instead of D=4*1 mm=4 mm), or the condition that each z position of the examination object must be irradiated during all n (in this case n=5) cardiac cycles is relinquished. The measurement data required at a specific z position then have to be generated by so-called spiral interpolations—known per se—from measurement data situated remote from the image plane. Both cases (larger layer thickness and lengthier spiral interpolation) result in a loss of sharpness in the z direction, which is undesirable for imaging fine object structures such as e.g. the coronary arteries. Although images with good temporal resolution are then obtained, they have inadequate spatial resolution.